The present invention relates to an interface circuit between a microcomputer and a driving circuit for controlling an inverter system by means of a microcomputer.
In recent years, in an inverter system to be used for motor control or the like, for example, control with a microcomputer is generally applied due to a demand for functional improvement. In this technique, a more acceptable interface circuit configuration is required between the microcomputer and a driving circuit.
Conventionally, as shown in FIG. 3, a photo coupler 24 is generally used for insulation between a driving circuit 25 of a semiconductor power element and a control signal output side of a microcomputer 22. As the photo coupler 24, an open collector output type of photo coupler is mostly used, and in this output type, an output signal from the photo coupler 24 is inverted logically with respect to its input signal.
FIGS. 4A through 4C show a square waveform representing an output voltage signal Vom of the microcomputer in the conventional interface circuit configuration between the microcomputer and the driving circuit, and a square waveform representing an input voltage signal Vin of the driving circuit and a square waveform representing an output voltage signal Vo of the driving circuit. In the interface circuit configuration between the microcomputer and the driving circuit, when the input voltage signal Vin of the driving circuit 25 is in a low level, the output voltage signal Vo of the driving signal 25 is brought into a high level so that the output voltage signal Vom of the microcomputer 22 and the output voltage signal Vo of the driving circuit 25, namely, an input voltage signal to the semiconductor power element are homeomorphous signals (so-called low-active operation).
Incidentally, in the case where HVIC (high Voltage IC) is applied as the driving circuit of the semiconductor power element, as shown in FIG. 5, the photo coupler is not used and the driving circuit can be directly connected with the microcomputer. In this configuration, one end of a resistor R3 is connected with a line which connects the driving circuit 25 and the microcomputer 22. Moreover, a pull-up power supply voltage Ep is provided on the other end of the resistor R3.
However, in the current situation, also in the case where HVIC is applied as the driving circuit, a configuration using the photo coupler (see FIG. 3) similarly to the conventional technique, or a configuration where, as shown in FIG. 5, the driving circuit 25 is directly connected with the microcomputer 22 and a control signal is inverted to be output on the side of the microcomputer 22 is used. As a result, the low-active system is mostly adopted in the driving circuit 25. In the former configuration, a characteristic of the high-withstand voltage HVIC that the photo computer is not necessary cannot be utilized. On the other hand, in the latter configuration, in the case where the conventional configuration of the low-active system driving circuit is directly adopted, when the pull-up power supply voltage Ep is higher than a power supply voltage Em of the microcomputer 22, an excess current flows from the pull-up power supply voltage Ep into a collector of the microcomputer 22 via the pull-up resistor R3. As a result, the microcomputer 22 is possibly broken.
The present invention has been developed in view of the above technical drawback and its object to provide an interface having higher reliability between a driving circuit of a semiconductor power element and a microcomputer for controlling output of the driving circuit.
In an aspect of the invention, there is provided an interface between a high-active system driving circuit for driving a predetermined semiconductor power element and a microcomputer for controlling an output signal of the driving circuit, the interface for directly connecting an output side of the microcomputer and an input side of the driving circuit, characterized in that: the microcomputer comprises a transistor having a collector terminal to be the output side of the microcomputer, and the driving circuit comprises: a first resistor, one end of which is directly connected with the output side of the microcomputer; a Schmidt circuit connected in a series with the other end of the first resistor; a diode, an anode side of which is connected between the first resistor and the Schmidt circuit; a power supply voltage connected with a cathode side of the diode; and a second resistor, one end of which is grounded and the other end of which is connected with a side of the first resistor being an input terminal of the driving circuit.